The present invention relates to tunable pre-selection filters, and particularly to methods and apparatus for tuning pre-selection filters in radio receivers.
Radio receivers are designed to receive modulated signals (e.g., amplitude modulated (AM), frequency modulated (FM), and 8-symbol phase shift keying (8-PSK) signals) centered at particular carrier frequencies. In typical broadcast systems, a broad band of carrier frequencies is typically divided up into a number of adjacent channels, each centered at a unique carrier frequency and having its own associated narrow bandwidth. The adjacent channels are designed not to overlap one another, in order to avoid interference between neighboring channels.
When a radio receiver is tuned to a particular one of these channels, it needs to be selectively responsive to the radio signals within the narrow bandwidth centered at the channel's center frequency. At the same time, the radio receiver needs to be capable of rejecting (i.e., being substantially non-responsive to) signals falling outside of its narrow frequency band.
Although the radio receiver is tuned to receive a channel at a particular carrier frequency, this high frequency signal (referred to as “radio frequency”, or RF) is typically converted to a lower frequency, or “baseband”, signal before the information modulated onto the signal is extracted and processed. This frequency conversion is typically performed by means of mixers, which mix the received RF signal with another signal. The RF signal (having a given carrier frequency) may be converted directly to the baseband signal by mixing the received RF signal with a signal oscillating at the same carrier frequency. Receivers that operate in this fashion are called “homodyne” receivers.
It is often desirable to convert the RF signal down to the baseband signal in incremental steps, rather than in one step. In such cases, the RF signal may first be converted into one or more so-called “intermediate frequency” (IF) signals, which are centered at respective frequencies lying somewhere in-between those of the RF signal and the baseband signal. Receivers that operate in this fashion are called “heterodyne” receivers.
Generation of an IF signal may be accomplished by mixing the original RF signal with a locally generated signal oscillating at a different carrier frequency. The resultant IF signal will carry the desired information on an oscillating signal whose center frequency is related to the difference between the RF carrier frequency and the locally generated signal. Because it is usually desired to generate an IF signal whose frequency is fixed, regardless of the carrier frequency of the received RF signal, receivers are designed such that the difference between the received RF carrier frequency and the frequency of the locally-generated signal will be maintained at a constant value. For example, as the front-end of the receiver is adjusted to receive a higher/lower RF carrier signal, the generator of the locally-generated signal is correspondingly adjusted to generate a higher/lower frequency signal, such that the difference between the two frequencies does not change.
Regardless of the type, a receiver needs to be capable of withstanding the presence of strong interfering signals within the same frequency band as the desired received signal. However, in many radio environments such as that found in mobile telecommunications, there can exist interfering signals that are only a few megahertz (MHZ) away from the desired signal. Furthermore, these interfering signals can sometimes be several orders of magnitude stronger than the desired signal.
To mitigate the effects of such strong nearby interfering signals, while simultaneously achieving as good a dynamic range as possible, a receiver is often a compromise between design choices favoring small signal properties (e.g., low noise characteristics) and other design choices favoring large signal properties (e.g., intercept point and signal compression). To illustrate this point, several conventional receivers will be described.
FIG. 1 is a block diagram of a conventional single band homodyne receiver. An RF signal is received by an antenna 101 and supplied to a band-pass filter 103 that suppresses all out-of-band interferers so that they will not exceed the level of the in-band interferers. This is done in order to prevent blocking of the receiver. In the exemplary embodiment, the desired frequency band is the range from 1805 to 1880 MHZ. The band-pass filter thus acts as a band selection filter, also known as a pre-selection filter or blocking filter.
From the output of the band-pass filter 103, the received signal is supplied to a low noise amplifier 105. After amplification, the signal is down-converted to respective in-phase (I) and quadrature (Q) baseband signals by first and second mixers 107, 109. This is accomplished by mixing the amplified received signal with respective locally-generated signals that each oscillate at the desired RF frequency, but which are 90 degrees out of phase with respect to one another. The purpose of separating the received signal into the I and Q baseband signals is to facilitate the demodulation of the signal (i.e., the extraction of the underlying information carried by the received signal). This aspect of the receiver operation is well-known, and need not be discussed here in further detail.
The respective locally-generated signals for use by the first and second mixers 107, 109 are created by first using a local oscillator circuit 125 to generate a signal of the desired frequency. The local oscillator circuit 125 is often implemented as a phase-locked loop (PLL). The signal from the local oscillator circuit 125 is then supplied to a phase-shifting circuit 111 that shifts the phase of the locally-generated signal by 90 degrees. The original (non-shifted) signal may then be supplied to the first mixer 107, while the phase-shifted signal may be supplied to the second mixer 109.
After down-conversion, the I and Q baseband signals are supplied to respective first and second channel selection filters 113, 115. The pass-band of each of these channel selection filters 113, 115 is much narrower than that of the band selection filter 103 because it is used to separate the received signal from the in-band interferers. After channel selection, the resultant I and Q signals could be subjected to further filtering and amplification, e.g., by respective first and second amplifiers 117, 119. In digital environments, the resultant analog signals may be converted into digital form by respective first and second analog-to-digital (A/D) converters 121, 123.
At this point it should be noted that, in the case of a heterodyne receiver, an extra mixing stage (not shown) would be disposed between the output of the low noise amplifier 105 and the inputs of the first and second mixers 107, 109. The extra mixing stage would generate an IF signal by mixing the originally received RF signal with a locally-generated signal that oscillates at a frequency that differs from the carrier frequency of the RF signal by a known amount. A channel selection filter may then operate on the IF signal, and its output supplied to the first and second mixers 107, 109 for a second down-conversion to the baseband frequency. In this case, the frequency of the locally-generated signals respectively supplied to the first and second mixers 107, 109 would be designed to match the frequency of the IF signal, rather than the frequency of the RF signal.
The active parts of the receiver, such as the low noise amplifier 105 and mixers 107, 109, are designed to exhibit good noise properties while also being able to withstand strong signals without degrading performance for weak signals. Consequently, the design will always be a trade-off between considerations relating to noise, linearity, and power consumption.
In many applications, it is desirable to have a radio receiver that is capable of operating in any of a number of distinct frequency bands. For example, a cellular telephone may be designed to operate in accordance with any of a number of different standards, each operating within a distinct frequency band. FIG. 2 is a block diagram of a conventional dual-band homodyne receiver that is capable of receiving signals in either of two frequency bands: a first band ranging from 1805 to 1880 MHZ, and a second band ranging from 1930 to 1990 MHZ. In order to enable the reception of two distinct frequency bands, the front-end of the receiver includes two distinct paths. In a first path, a first band-pass filter 201 is designed to suppress frequencies outside the range from 1805 to 1880 MHZ. The resultant signal is supplied to a first low noise amplifier 203. Similarly, in a second path of the front-end of the receiver, a second band-pass filter 205 is designed to suppress frequencies outside the range from 1930 to 1990 MHZ. The resultant signal from the second band-pass filter 205 is supplied to a second low noise amplifier 207. Selection of the desired frequency band may be accomplished by controlling the first and second low noise amplifiers 203, 207 in such a way that only one of them supplies an output to the remaining components of the receiver. These remaining components operate in the same way as the counterparts described above with respect to the single band receiver depicted in FIG. 1.
A problem with the above-described receivers is that strong in-band interferers may pass through the band selection filter without any suppression. These in-band interferers must first be amplified and down-converted before they can be suppressed by any channel selection filtering (e.g., by the channel selection filters 113, 115). These in-band interferers put very high linearity requirements on the front-end part of the receiver in order to avoid desensitization due to:                1. Strong signals driving the front-end into compression and thereby degrading the signal-to-noise ratio (SNR) in the receiver.        2. Strong signals causing reciprocal mixing of local oscillator phase noise.        3. Strong signals causing distortion through intermodulation caused by second or third order distortion (IP2, IP3). Second order distortion products due to AM interferers are a well-known problem in homodyne or low-IF receivers.        
Another problem associated with multi-band receivers (e.g., the dual-band receiver illustrated in FIG. 2) is that these receivers add extra filters and switching mechanisms, even if the receive bands are relatively close, as in the DCS 1800 and PCS 1900 cellular communication systems. These extra components increase the complexity and cost of the receiver. The additional band switching devices also degrade the noise performance of the receiver due to the increased insertion loss between the antenna and the receiver front-end.
As a solution to the above identified problems, it has been proposed to move some of the channel selectivity to the filter preceding the front-end. For example, U.S. Pat. No. 5,065,453 discloses an electrically-tunable band-pass filter for providing front-end selectivity in a superheterodyne radio receiver. The band-pass filter provides a narrow front-end filter which is tuned automatically as the local oscillator frequency is changed.
U.S. Pat. No. 5,752,179 discloses a selective RF circuit with varactor tuned and switched band-pass filters. In this arrangement, low-, mid- and high-band-pass filters are selectively activated to cover a tuning range of the receiver. Each of these three filters is, itself, tunable when activated.
U.S. Pat. No. 5,150,085 discloses an electronically tunable front-end filter for use in a radio apparatus. The filter includes a plurality of isolated ceramic resonators, each having an associated varicap diode network to enable electronic tuning respective of ceramic resonators.
JP 2170627 A discloses a tunable filter interposed between two integrated circuits (ICs). The first of the ICs is an RF amplifier, while the second of the ICs is a mixer. The tunable filter is tuned by interlocking with a tuning voltage of an oscillating circuit.
Since, in these arrangements, the front-end filter acts as a band selection filter, it must be tunable to be able to select any channel within the receiver band. The tuning of this tunable filter must then be arranged in some clever way in order not to degrade performance for the received signal. That is, the tuning must always result in the best possible receiver for the received signal and at the same time offer some attenuation of strong in-band interferers located some channels away from the received signal.
Thus, there are very severe tuning requirements placed on the tunable front-end filter. However, it is difficult to tune these filters to the correct frequency because of spread in component values and because of temperature-related drift of the filter's center frequency. This is conventionally solved by production trimming, which is very time consuming if it has to be performed for all temperatures. Another problem with trimming only once in a factory is that this trim value remains constant while the tunable front-end filter changes its characteristics due to aging, temperature drift and/or moisture, which changes cannot be measured. Consequently, the receiver's performance degrades over time.